1. Field of the Invention
The present invention relates to water injection type screw compressors including a pair of intermeshing male and female screw rotors and using water in place of oil as a liquid coolant.
2. Description of the Related Art
Conventionally, as compressors which compress a gas such as air for use as compressed gas, so-called oil cooled compressors (oil cooled screw compressors) have often been used. In such an oil cooled compressor, it is necessary to prevent increases in temperature of compressed air or other components resulting from heat generated during compression, and seal gaps between machine elements, such as screw rotors and a rotor casing, in a compression working space which is a region to generate compressed air. Due to the necessities, oil is introduced into the compression working space where compression is in progress, and supplied to a speed increasing gear unit or roller bearings.
Typically, in the compressors of the oil cooled type, an apparatus for separating an oil component, such as an oil separator, is additionally provided in a flow channel on a discharge side. Specifically, although compressed gas that contains the oil component is discharged from the oil cooled compressor, the oil component is removed in the above-described apparatus for separating an oil component, such as the oil separator, to thereby provide the compressed gas containing no oil component to a device in which the compressed gas is supplied.
In practice, however, there is a great difficulty in completely removing the oil component. Therefore, the oil cooled compressors cannot be used in factories, such as food factories, pharmaceutical factories, precision machine factories, etc. where a supply of clean, oil free compressed air is required. For this reason, water injection type compressors (water lubricated compressors) which use water in place of oil have been suggested (refer to, for example, JP 2007-162484 A), and they are becoming widely used.
Because the compressed air generated by the water injection type compressor includes no oil component, the water injection type compressor can be used even in the factories that need clean compressed air. However, in a case where the water injection type compressor is a so-called screw compressor, when male and female screw rotors housed in a compression space of that water injection type screw compressor are made of metal, it is not possible to directly engage the male and female screw rotors with each other even under a condition that water is supplied to the compression space. This is because a viscosity of water is lower than that of oil.
Thus, in a typical water injection type screw compressor, timing gears are mounted on end regions of the male and female screw rotors to rotate the screw rotors by means of the timing gears while maintaining a gap between the male and female screw rotors. The timing gears, a speed increasing gear unit, and bearings for supporting the screw rotors (such as roller bearings) cannot be lubricated with water, and instead should be lubricated with oil. As a scheme for lubricating the components such as the timing gears with oil, an oil bath system for storing oil in a housing that includes the components such as the timing gears and at least partially soaking the component in the oil, or a forced circulation system for forcibly circulating oil through the components may be employed.
On the other hand, for the purpose of reducing (eliminating the need for) lubrication with oil in the water lubricated screw compressor, it is necessary to use the screw rotors, which are formed of resin and allowed to directly engage with each other even in an environment where water having a low viscosity is present, to employ a water lubricated slide bearing as a bearing for supporting the screw rotors, and to take other measures.
However, the water lubricated screw compressors have problems as described below. For example, in a type requiring oil lubrication of components such as the timing gears, when the number of rotations of the timing gears or other components is increased in the above-described oil bath system, an agitation loss becomes greater. Such a greater agitation loss produces an increase in oil temperature, resulting in poor lubrication or a reduced life of lubrication oil. In addition, because the oil is not exchanged in the above type, if a foreign object is introduced, it is not easy to remove the foreign object.
On the other hand, in the above-described forced circulation system, the agitation loss such as that occurs in the oil bath system is small because oil is forcibly circulated by means of an oil circulating pump or the like. Further, an oil filter can be attached at some midpoint of a pipe through which the oil is circulated, to thereby supply the timing gears and other components with clean oil at any time, which secures a higher degree of reliability as compared to the oil bath system. It is however necessary that a cooling means, such as an oil cooler, should be inserted in an oil circulating channel to suppress the increase in oil temperature. Thus, the forced circulation system still has room for improvement in terms of a downsizing of equipment, reduction in cost, and the like.
Moreover, even the water lubricated screw compressor of the type arranged to reduce (eliminate the need for) the lubrication with oil has a problem as described below. That is, the screw rotor formed of resin has a liner expansion coefficient which is higher than that of a screw rotor formed of metal, and thus tends to expand while absorbing water over time. This requires a relatively greater gap to be previously defined between the male and female screw rotors, resulting in a low degree of compression efficiency. Meanwhile, the water lubricated slide bearing, which typically has a clearance greater than that of a ball bearing, is inferior in performance as a bearing, and might not be suitably resistant to wear. In general, a combination of the resin screw rotors and the water lubricated slide bearing is more expensive than a combination of the metallic screw rotors and the ball bearing, which also leaves room for improvement in terms of cost reduction.
Therefore, it is an object of the present invention to provide water lubricated screw compressors, in particular, a water lubricated screw compressor of a type which requires oil lubrication for components in a forced lubrication system, the structure of the water lubricated screw compressor being simplified by eliminating an oil cooling means such as an oil cooler, while an increase in oil temperature is minimized.